Method and apparatus for transmitter optimization based on allocated transmission band

ABSTRACT

First and second inputs are received. The first input indicates a frequency offset of a frequency band allocated for signal transmission. The said allocated band is a subband of a total band available for transmission. The second input indicates a bandwidth of the allocated band. One or more filters of a transmitter of a communications system are controlled to operate cumulatively in a lowpass filtering mode, wherein the highest frequency in a pass band in the lowpass filtering mode is less than the highest frequency of the total band available for transmission. A signal is filtered using the filter(s).

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/696,265, entitled “Method and Apparatus for Transmitter OptimizationBased on Allocated Transmission Band,” filed Apr. 24, 2015 which is adivisional of U.S. patent application Ser. No. 13/730,127, entitled“Method and Apparatus for Transmitter Optimization Based on AllocatedTransmission Band,” filed Dec. 28, 2012, issued as U.S. Pat. No.9,031,567, each of which are hereby incorporated by reference in theirentirety.

FIELD

The present disclosure relates to signal processing for wirelesscommunications, and more particularly, some embodiments relate tomethods and systems for transmitter optimizations based on informationabout a frequency band allocated for transmission of a signal.

BACKGROUND

With the ever-increasing prevalence of wireless communications,efficient techniques for signal processing associated with suchcommunications are of great importance. Several emerging wirelesscommunication standards such as WiMax and LTE (Long Term Evolution) havebeen gaining in prominence, and radio frequency (RF) transceivers areused to transmit OFDMA (Orthogonal Frequency Division Multiple Access)and SC-FDMA (Single Carrier Frequency Division Multiple Access) signalsto support such standards. FIG. 1 is an illustration of a typicalwireless communication system. A transmitter provides a signal to atransmit antenna 104, which transmits the signal over a channel 106,which may include air. A receive antenna 108 receives the signal andprovides it to a receiver 110 for processing. Antennas 104 and 108 maybe considered as part of the transmitter and receiver, respectively, insome configurations. Processing that occurs at transmitter 102 preparesan input signal, e.g., by modulating data and preparing the signal forRF transmission. In a typical direct conversion transmitterimplementation, a local oscillator (LO) signal is tuned to match adesired RF signal that is intended for transmission, so that thebaseband quadrature signal is converted directly to RF. At baseband,digital-to-analog conversion and various other processing stagesgenerate noise and distortion. Filter circuits are commonly provided toaddress such noise and distortion prior to signal transmission.

SUMMARY

In some embodiments of the present disclosure, first and second inputsare received. The first input indicates a frequency offset of afrequency band allocated for signal transmission. The said allocatedband is a subband of a total band available for transmission. The secondinput indicates a bandwidth of the allocated band. A test is performedto determine whether the frequency offset is greater than a firstthreshold. Responsive to a determination that the frequency offset isgreater than the first threshold, one or more filters of a transmitterof a communications system are controlled to operate cumulatively in abandpass filtering mode, wherein the frequency offset is within a passband in the bandpass filtering mode and the pass band is at least aswide as the allocated band. A signal is filtered using the filter(s).

In some embodiments, first and second inputs are received. The firstinput indicates a frequency offset of a frequency band allocated forsignal transmission. The said allocated band is a subband of a totalband available for transmission. The second input indicates a bandwidthof the allocated band. One or more filters of a transmitter of acommunications system are controlled to operate cumulatively in alowpass filtering mode, wherein the highest frequency in a pass band inthe lowpass filtering mode is less than the highest frequency of thetotal band available for transmission. A signal is filtered using thefilter(s).

In some embodiments, first and second inputs are received. The firstinput indicates a frequency offset of a frequency band allocated forsignal transmission. The said allocated band is a subband of a totalband available for transmission. The second input indicates a bandwidthof the allocated band. A local oscillator of a transmitter of thecommunications system is re-tuned from a first frequency to a secondfrequency to move spectral content of a carrier-modulated signal tobaseband or closer to baseband. One or more filters of the transmitterare controlled to operate cumulatively in a lowpass filtering mode. Thesignal is filtered using the filter(s).

In some embodiments, a system includes a local oscillator, one or morevariable bandwidth filters arranged along a serial processing pathway, amixer, and a control module. The local oscillator is configured tosynthesize a waveform at a variable frequency. Each filter is variablyconfigurable to operate in either a bandpass filtering mode or a lowpassfiltering mode, and each filter is capable of being enabled or disabled.The mixer is configured to mix an output of one of the filters based onthe waveform synthesized by the local oscillator. The control modulecomprises a computer readable storage medium includingcomputer-executable instructions stored tangibly thereon. When executed,the instructions cause a processor of the system to perform theoperations of: receiving a first input and a second input, wherein thefirst input indicates a frequency offset of a frequency band allocatedfor signal transmission, said allocated band is a subband of a totalband available for transmission, and the second input indicates abandwidth of said allocated band; determining whether the frequencyoffset is greater than a first threshold; and responsive to adetermination that the frequency offset is greater than the firstthreshold, controlling the one or more filters to operate cumulativelyin bandpass mode, wherein the frequency offset is within a pass band inthe bandpass filtering mode and said pass band is at least as wide asthe allocated band.

In some embodiments, a system includes a local oscillator, one or morevariable bandwidth filters arranged along a serial processing pathway, amixer, and a control module. The local oscillator is configured tosynthesize a waveform at a variable frequency. Each filter is capable ofbeing enabled or disabled. The mixer is configured to mix an output ofone of the filters based on the waveform synthesized by the localoscillator. The control module comprises a computer readable storagemedium including computer-executable instructions stored tangiblythereon. When executed, the instructions cause a processor of the systemto perform the operations of: receiving a first input and a secondinput, wherein the first input indicates a frequency offset of afrequency band allocated for signal transmission, said allocated band isa subband of a total band available for transmission, and the secondinput indicates a bandwidth of said allocated band; and controlling theone or more filters of a transmitter of the communications system tooperate cumulatively in lowpass filtering mode, wherein the highestfrequency in a pass band in said lowpass filtering mode is less than thehighest frequency of the total band available for transmission.

In some embodiments, a system includes a local oscillator, one or morevariable bandwidth filters arranged along a serial processing pathway, amixer, and a control module. The local oscillator is configured tosynthesize a waveform at a variable frequency. Each filter is variablyconfigurable to operate in either a bandpass filtering mode or a lowpassfiltering mode, and each filter is capable of being enabled or disabled.The mixer is configured to mix an output of one of the filters based onthe waveform synthesized by the local oscillator. The control modulecomprises a computer readable storage medium includingcomputer-executable instructions stored tangibly thereon. When executed,the instructions cause a processor of the system to perform theoperations of: receiving a first input and a second input, wherein thefirst input indicates a frequency offset of a frequency band allocatedfor signal transmission, said allocated band is a subband of a totalband available for transmission, and the second input indicates abandwidth of said allocated band; determining whether sufficient timeremains for retuning a local oscillator of a transmitter of thecommunications system before a scheduled transmission time; andresponsive to a determination that sufficient time remains for retuningthe local oscillator before the scheduled transmission time, retuningthe local oscillator from a first frequency to a second frequency andcontrolling one or more filters of the transmitter to operatecumulatively in a lowpass filtering mode.

BRIEF DESCRIPTION OF THE DRAWINGS

The following will be apparent from elements of the figures, which areprovided for illustrative purposes and are not necessarily to scale.

FIG. 1 is an illustration of a conventional communications system.

FIG. 2 is an illustration of spectral content at a transmitter for anentire bandwidth usage case.

FIG. 3 is an illustration of spectral content at a transmitter for apartial bandwidth usage case.

FIG. 4 is a block diagram of a transmitter architecture in accordancewith some embodiments of the present disclosure.

FIG. 5 is an illustration of a frequency response in accordance with oneexample embodiment.

FIG. 6 is an illustration of a frequency response in accordance withanother example embodiment.

FIG. 7 is an illustration of a frequency response in accordance withanother example embodiment.

FIG. 8 is an illustration of a frequency response in accordance withanother example embodiment.

FIG. 9 is an illustration of a frequency response in accordance withanother example embodiment.

FIG. 10 is an illustration of spectrum allocation which may be used inaccordance with some embodiments.

FIGS. 11A-11B are flow charts of example processes in accordance withsome embodiments.

FIG. 12 is a flow chart of another process in accordance with someembodiments.

FIG. 13 is a flow chart of another process in accordance with someembodiments.

FIG. 14 is a flow chart of another process in accordance with someembodiments.

DETAILED DESCRIPTION

This description of the exemplary embodiments is intended to be read inconnection with the accompanying drawings, which are to be consideredpart of the entire written description.

FIG. 2 is an illustration of a frequency spectrum at a transmitter,e.g., in an OFDMA or SC-FDMA context. In this example, a signal havingspectral content 210 that is desired to be transmitted occupies theentire bandwidth that is available for transmission. This signal isshown centered about a tuned local oscillator frequency. Out-of-bandgenerated noise and distortion is shown as 220 in FIG. 1 and may includeDAC (digital-to-analog converter) alias components 225. It is desirableto transmit the signal with low out of band noise and distortion, assuch noise and distortion that is transmitted outside the used frequencyspectrum can degrade overall system performance. For this reason,various devices (e.g., cellular handsets) have to reduce this out ofband noise and distortion below specified levels. High-order filters(e.g., having filter characteristic 214 shown in FIG. 2) are typicallyused to provide steep transitions between pass bands and reject bands inorder to transmit the desired signal while at the same time rejectinginterference. Such high-order filters having steep transitions aregenerally complex and expensive.

Another consideration for wireless system operation is that the signalto be transmitted by a particular user may not occupy the entirebandwidth that is available for transmission. For example, FIG. 3 showsa scenario where the desired signal to be transmitted has spectralcontent 310 which is only a subset of the entire bandwidth denoted by312. The corresponding unwanted out of band spectral content 320including alias components 325 are also shown in FIG. 3. Thus, basebandcircuits are often designed to transmit the entire bandwidth 312 even ifmost of the time the particular user is only receiving a signal such assignal 310 that occupies only part of the total bandwidth 312.Typically, prior art architectures have used the same steep-transitionfilter for the partial bandwidth usage case of FIG. 3 as for the entirebandwidth usage case of FIG. 2, which is an inefficient use ofresources.

Various embodiments of the present disclosure address the foregoinginefficiencies by optimizing the transmitter configuration dynamically,e.g., based on dynamic allocation of frequency bands within the totalavailable band.

FIG. 4 is a block diagram of a transmitter architecture in accordancewith some embodiments of the present disclosure. A receiver 410 is shownat the top of FIG. 4, components of a transmitter are shown at thebottom, and a digital baseband module 420 is shown at the right. Varioussignals are shown as differential signals in FIG. 4, althoughsingle-ended implementations may be used as well. An RF configurationcontrol module 430 in the digital baseband module is aware of thebandwidth being used for the desired signal for every allocation (e.g.,every OFDMA or SC-FDMA allocation if either of those standards isapplicable for the wireless communication system). Based on thisinformation, control module 430 may send control signals 450 a, 450 b,450 c (generally control signals 450) to program the RF circuitry forefficient transmission. Control module 430 may include a processor and anon-transitory computer readable storage medium having instructionstangibly embodied thereon that, when executed, cause the processor toperform various processing.

Signal 450 a is sent to a synthesizer RF circuit (local oscillator) 444,which may be tuned so that various frequencies may be transmitted. Thegenerated waveform 446 is provided to a mixer 436, which may include anin-phase mixer component and a quadrature mixer component. The mixer 436may receive in-phase and quadrature inputs. A balancer 438 and amplifier440 may be provided to yield signal 442.

One or more filters are provided in the transmitter. FIG. 4 shows anexample with two filters BBF1 and BBF2, although other numbers of filterstages are possible. FIG. 4 shows filters (and other components) inin-phase and quadrature processing paths; in some implementations, asingle processing path may be provided. The filters are configurable inbandwidth and can also be set to either low pass filter (LPF) or bandpass filter (BPF) mode, or they can be selectively bypassed. Controlsignals 450 b and 450 c control the configuration of filters BBF1 andBBF2, respectively. This flexibility in filter settings will allowseveral configuration options described in Table 1.

TABLE 1 Example filter configuration options BBF1 configuration BBF2configuration Option 1 Low Pass Low Pass Option 2 Low Pass Band PassOption 3 Band Pass Band Pass Option 4 Band Pass Low Pass Option 5 BypassLow Pass Option 6 Bypass Band Pass Option 7 Low Pass Bypass Option 8Band Pass Bypass

The individual configurations of BBF1 and BBF2 determine the overall(cumulative) filter mode of the transmitter, e.g., because configuringany individual filter stage in bandpass mode causes the overall (final)filter response to be bandpass.

FIG. 5 is an illustration of a frequency response in accordance with oneexample embodiment. FIG. 5 shows a frequency content of signal 510desired to be transmitted, noise/distortion 520, and DAC alias component525. The frequencies corresponding to region 510 may be referred to asan allocated band, and the center frequency of region 510 may bereferred to as a frequency offset f_(offset). As in FIG. 3, frequencycontent of signal 510 occupies only a portion of the total bandwidth312. The filter that is cumulatively formed by filters BBF1 and BBF2 maybe configured to operate as a bandpass filter. This may be accomplishedvia options 2, 3, 4, 6, or 8 of Table 1, because when any filter stageis in bandpass mode, the final filter response is bandpass. The filter'sbandwidth can be reduced compared to bandwidth of the filter in FIG. 3,and the filter order can also be reduced, as shown by filtercharacteristic 514, compared to the filter characteristic 314 of FIG. 3.The filter bandwidth can be reduced so that only the frequencies of thedesired signal 510 are passed by the pass band of the filter. Thereduction in filter order allows potentially less stages of filtering tobe used. So, in some embodiments, only BBF1 is used in a bandpass modeto reject the out of band noise, distortion, and/or DAC alias. Byreducing the filter order, current drain is saved and distortion to thedesired signal 510 is reduced.

FIG. 6 is an illustration of a frequency response in accordance withanother example embodiment. FIG. 6 shows a frequency content of signal510 desired to be transmitted, noise/distortion 520, and DAC aliascomponent 525. For this example, the LO (local oscillator) in the RFcircuits is retuned to center signal 510 at baseband (i.e., retune tof_(offset)) and then a lower order baseband low pass filter (LPF) isused to reject the out of band noise and distortion. Thus, the LO isretuned from frequency 630 to frequency 632. As long as the carrierfeedthrough of the RF LO can be adequately reduced, this allowstransmission with a lower order filter (lower order than in FIG. 3)having filter characteristic 614, thus yielding the benefits previouslydescribed. One of ordinary skill in the art will understand thatfeedthrough refers to LO signal leakage to the output of thetransmitter, which corrupts the transmitter's output signal. Suchleakage can result from imperfections (mismatches) of mixer 436 andreduces signal to noise ratio (SNR) of the transmitted signal. In theexample shown in FIG. 6, the bandwidth of the lowpass filter is set tothe transmission bandwidth, i.e., the bandwidth of signal 510. Thelowpass filter of the example of FIG. 6 may be configured by usingoptions 1, 5, or 7 of Table 1 for the individual filters BBF1 and BBF2,since those options involve setting each filter stage to lowpass.

FIG. 7 is an illustration of a frequency response in accordance withanother example embodiment. In this example, the LO is retuned tofrequency 730 and a lower bandwidth bandpass filter (BPF) (lowerbandwidth than in FIG. 3) is used. For this example, the LO can beretuned so that it does not create LO feedthrough in the allocated bandbut reduces the frequency offset of the desired signal at baseband. Thismay be accomplished by retuning the LO to frequency 730 which is lessthan the lowest frequency of the allocated band. It is also possible tonot retune the LO and only change the bandwidth and mode (i.e., changeto bandpass mode) of the filter if there is not enough time for retuningthe LO before the signal 510 is scheduled to be transmitted. The filteris reconfigured to a lower order (than in FIG. 3) BPF having filtercharacteristic 714 in order to reject the noise and distortion. Again,this allows transmission with a lower order filter (implementable byoptions 2, 3, 4, 6, or 8 of Table 1) thus yielding the benefitsdescribed above.

FIG. 8 is an illustration of a frequency response in accordance withanother example embodiment. In this example, the LO is retuned fromfrequency 630 to frequency 730 and a lower bandwidth LPF (than in FIG.3) is used. For this case, the LO is retuned so that it does not createLO feedthrough in the allocated band but reduces the frequency offset ofthe desired signal at baseband. The filter of FIG. 8 may be implementedby options 1, 5, or 7 of Table 1.

FIG. 9 is an illustration of a frequency response in accordance withanother example embodiment. In this example, the LO is at frequency 630and is not retuned, e.g., if there is not enough time to retune the LObefore the signal is scheduled to be transmitted. The filter isreconfigured to a lower order LPF (lower order than in FIG. 3) in orderto reject the noise and distortion. The bandwidth of the filter is lowerthan in FIG. 3, e.g., with the highest frequency in the pass band of theLPF equaling the highest frequency of the allocated band. Again, thisallows transmission with a lower order filter (implementable by options1, 5, or 7 of Table 1) thus yielding the benefits described above.

The configurations of FIGS. 5-9 may all be achieved using the samehardware and by providing suitable control signals 250. Variousadvantages and disadvantages of the respective configurations are nowdescribed.

The approach of FIG. 5 leaves LO feedthrough at 0 Hz. There is nomodulated signal present there (signal 510 does not contain content at 0Hz) and therefore the emissions requirement is relaxed. The filterbandwidth can be equal to the used bandwidth. LO retuning is notrequired, which is advantageous because time is required to retune. Onthe other hand, bandpass filters are more complex to implement than lowpass filters, and if the transmission spectrum is at band edge, morecurrent is needed due to the high frequency offset.

Regarding the configuration of FIG. 6, the low pass filter is a simplerimplementation than a bandpass filter. The transmitted spectrum may bekept at the lowest possible frequency in the baseband, allowing minimumcurrent drain. The filter bandwidth can be equal to the used bandwidth.On the other hand, LO feedthrough falls inside the modulated signalbandwidth and must be kept low or it will affect the signal quality.And, LO retuning is used, which means there must be enough time toretune before the next transmission.

Regarding the configuration of FIG. 7, LO feedthrough is not inside themodulated signal bandwidth and will not degrade the signal quality. Thefilter bandwidth can be equal to the used bandwidth. On the other hand,LO feedthrough is not at 0 Hz and must be kept low enough to meetemissions requirements, and the bandpass filter is more complex toimplement than a lowpass filter.

In the configuration of FIG. 8, the low pass filter is a simplerimplementation than a bandpass filter. LO feedthrough is not inside themodulated signal bandwidth and will not degrade the signal quality. Onthe other hand, LO feedthrough is not at 0 Hz and must be kept lowenough to meet emissions requirement, and the filter bandwidth must bewider than the used bandwidth.

Regarding the configuration of FIG. 9, this configuration is similar tothe configuration of FIG. 8 except LO retuning is not needed, whichenables this configuration to be used in a situation where there isinsufficient time to retune before the next transmission. On the otherhand, the filter bandwidth is wider in the FIG. 9 example than in theFIG. 8 example.

In order to illustrate the operation of the RF configuration controlmodule 430 of FIG. 4, a specific example is now described. This exampleuses LTE to describe how the RF configuration control may operate. InLTE, the spectrum used for transmission is divided into smaller blockscalled resource blocks (RBs) as illustrated in FIG. 10. A group ofcontiguous RBs are allocated to a particular user which enables the useof the variable transmitter control described herein.

For a specific example, consider the LTE case where 20 MHz of bandwidthis used. Due to the need for a guard band, the actual bandwidthavailable is 18 MHz, leaving 1 MHz of guard band on each side. For thiscase, the 18 MHz of bandwidth is divided into 100 RBs with 180 kHz perRB. These RBs can then be assigned to a particular user on a slot byslot basis. There are several example channels described in the 3GPP36.101 standard document for the 18 MHz case with different RBallocations. These are described in Table 2. The allocation of RBs maydetermine if reduced filtering can be used and whether some filteringcan be bypassed (e.g., whether to disable BBF1 or BBF2).

TABLE 2 Example RB allocations and rejection specifications 3-pole low1-pole low Trans- pass pass 3-pole low 1-pole low RBs mission rejectionat rejection at pass pass Allo- BW, 61.44 MHz, 61.44 MHz, rejection atrejection at cated MHz dB dB 30 MHz, dB 30 MHz, dB 1 0.18 150 56 150 5018 3.24 95 32 76 25 25 4.5 86 29 67 22 50 9 68 23 49 16 75 13.5 58 19 3813 100 18 50 16 31 10

Consider a first example where the DAC sample rate is 61.44 MHz. Toimplement the architecture shown in FIG. 4 with a 1-pole filter for BBF1and a 2-pole filter for BBF2, different filter configurations may beselected for BBF1 and BBF2 based on the RB allocation in order to reducethe level of the DAC alias. As a specific example, if the amount ofrejection required is 29 dB at the DAC alias (located at 61.44 MHz), theRB allocations with transmission BW<4.5 MHz (less than 25 RBs) willallow the system to operate with only BBF1 and bypass BBF2. Anotherfactor in the operation of RF configuration control module 430 is thelocation of the allocated RBs. The assigned RBs can be anywhere in thetransmission bandwidth from near DC to the band edge. This factor maydetermine whether a bandpass or lowpass filter mode is used for thefilter. Again as a specific example, assume that the amount of rejectionrequired is 29 dB at the DAC alias (located at 61.44 MHz). For a 1 RBallocation case, if the RB allocation is located at less than 4.5 MHzthen a lowpass filter can used, otherwise a bandpass filter is required.

As another second example, consider the specification for spectralemissions at a 30 MHz offset from the assigned channel. As with theprevious example, the architecture in FIG. 4 with a 1-pole filter forBBF1 and a 2-pole filter for BBF2 can be configured based on the RBallocation in order to reduce the baseband noise generated at a 30 MHzoffset. As a specific example, if 22 dB filter rejection is needed toreduce this noise, then if the RB allocation is located at less than 4.5MHz, a lowpass filter can be used. Otherwise, a bandpass filter is used.Another factor in the RF configuration control module is the amount oftime available to retune the LO before the transmission. If the time istoo short (e.g., less than 30 μsec), then it may not be possible toretune the LO and this will eliminate from consideration using some ofthe configurations described previously. Specifically, the techniques inFIGS. 6-8 are not possible in that case since LO retuning is assumed forthese configurations. The techniques of FIGS. 5 and 9 can be used forthis case. In one implementation of the RF configuration control module430, it is designed that the LO will always be retuned if possible inorder to reduce the filtering requirement. So the following threefactors may be used by the RF configuration control module 430 (and maybe provided as inputs to the control module) in order to optimize thesystem: (1) Bandwidth (RB allocation); (2) Location of RB allocation infrequency spectrum; (3) Time allowed for LO retuning.

FIG. 11A is a flow chart that illustrates how the RF configurationcontrol module 430 may operate using these data in accordance with someembodiments. The flow chart of FIG. 11A is only a depiction of oneexample implementation. The allocated transmission bandwidth andfrequency offset f_(offset) are received (block 1102), e.g., from a basestation. A check may be made as to whether there is sufficient time toretune the LO (block 1104). If so, the LO may be retuned, and the filter(e.g., formed by individual filters BBF1 and BBF2) is set to low passwith the filter bandwidth equal to the allocated transmission bandwidth.The LO may be retuned as in FIG. 6 or as in FIG. 8. If the LO is retunedas in FIG. 6, a check may be made as to whether LO feedthrough isunacceptably high (e.g., greater than a predetermined threshold), inwhich case the LO may be again retuned as in FIG. 8.

If there is not sufficient time to retune the LO, the process mayproceed to block 1106, where a comparison is made to check if f_(offset)exceeds a predetermined threshold. If it does exceed the threshold(block 1108), the filter (e.g., formed by individual filters BBF1 andBBF2) is set to bandpass mode at f_(offset) with the filter bandwidthequal to the transmission bandwidth, as in FIG. 5; otherwise (block1110), the filter is set to lowpass mode with the minimum bandwidth topass the desired signal with the given transmission bandwidth.

If the attenuation of BBF1 at a predetermined frequency (e.g., the DACalias frequency f_(DAC) or the frequency of a critical noiserequirement) exceeds a predetermined threshold (block 1112), BBF2 may bebypassed (block 1114) as described above, such that only BBF1 isenabled; otherwise, both BBF1 and BBF2 may be enabled (block 1116). Thesignal is then transmitted using the allocated spectrum.

In some embodiments, one or more of the tests 1104, 1106 are omitted.For example, the logic may be programmed to never retune the LO, or toalways retune regardless of how much time remains before the nexttransmission, so that the test at block 1104 can be omitted. As anotherexample, for which a flow chart is shown in FIG. 11B, the logic may beprogrammed to always perform the functionality of block 1108, oralternatively to always perform the functionality of block 1110, so thatthe test at block 1106 can be omitted.

FIG. 12 is a flow chart of a process in accordance with someembodiments. First and second inputs are received (block 1210). Thefirst input indicates a frequency offset of a frequency band allocatedfor signal transmission. The said allocated band is a subband of a totalband available for transmission. The second input indicates a bandwidthof the allocated band. A test is performed to determine whether thefrequency offset is greater than a first threshold (block 1220).Responsive to a determination that the frequency offset is greater thanthe first threshold, one or more filters of a transmitter of acommunications system are controlled (block 1230) to operatecumulatively in a bandpass filtering mode, wherein the frequency offsetis within a pass band in the bandpass filtering mode and the pass bandis at least as wide as the allocated band. A signal is filtered (block1240) using the filter(s).

FIG. 13 is a flow chart of a process in accordance with someembodiments. First and second inputs are received (block 1310). Thefirst input indicates a frequency offset of a frequency band allocatedfor signal transmission. The said allocated band is a subband of a totalband available for transmission. The second input indicates a bandwidthof the allocated band. One or more filters of a transmitter of acommunications system are controlled (block 1320) to operatecumulatively in a lowpass filtering mode, wherein the highest frequencyin a pass band in the lowpass filtering mode is less than the highestfrequency of the total band available for transmission. A signal isfiltered (block 1330) using the filter(s).

FIG. 14 is a flow chart of a process in accordance with someembodiments. First and second inputs are received (block 1410). Thefirst input indicates a frequency offset of a frequency band allocatedfor signal transmission. The said allocated band is a subband of a totalband available for transmission. The second input indicates a bandwidthof the allocated band. A local oscillator of a transmitter of thecommunications system is re-tuned (block 1420) from a first frequency toa second frequency to move spectral content of a carrier-modulatedsignal to baseband or closer to baseband. One or more filters of thetransmitter are controlled (block 1430) to operate cumulatively in alowpass filtering mode. The signal is filtered (block 1440) using thefilter(s).

Thus, various embodiments of the present disclosure use digital basebandawareness of the transmit bandwidth allocation to dynamically adjust theLO, the filter mode (lowpass or bandpass), or both, in order to optimizetransmitter performance. The transmitter performance is optimizedbecause a lower order filter with less current drain and less distortioncan be used to transmit the signal. Various embodiments are adaptablefor optimum transmission of any bandwidth allocation.

Although examples are illustrated and described herein, embodiments arenevertheless not limited to the details shown, since variousmodifications and structural changes may be made therein by those ofordinary skill within the scope and range of equivalents of the claims.

What is claimed is:
 1. A system comprising: one or more variablebandwidth filters arranged along a serial processing pathway, whereineach filter is capable of being enabled or disabled; a mixer configuredto mix an output of one of the one or more filters based on a waveformsynthesized by a local oscillator; and a control module comprising anon-transitory computer readable storage medium includingcomputer-executable instructions stored tangibly thereon, saidinstructions when executed causing a processor of the system to performthe operations of: receiving a first input and a second input, whereinthe first input indicates a frequency offset of a frequency bandallocated for signal transmission, said allocated band is a subband of atotal band available for transmission, and the second input indicates abandwidth of said allocated band; and controlling the one or morefilters of a transmitter of the communications system to operatecumulatively in filtering mode.
 2. The system of claim 1, wherein thehighest frequency in a pass band in said filtering mode is less than thehighest frequency of the total band available for transmission.
 3. Thesystem of claim 1, wherein said instructions when executed further causethe processor to: determine whether the frequency offset is greater thana first threshold; wherein said controlling the one or more filters isperformed upon the determination that the frequency offset is notgreater than the first threshold.
 4. The system of claim 1, wherein eachfilter is only operable in filtering mode.
 5. The system of claim 1,wherein each filter is variably configurable to operate in eitherbandpass filtering mode or lowpass filtering mode.
 6. A method ofoperating a wireless communications system, the method comprising:receiving a first input and a second input, wherein the first inputindicates a frequency offset of a frequency band allocated for signaltransmission, said allocated band is a subband of a total band availablefor transmission, and the second input indicates a bandwidth of saidallocated band; controlling one or more filters of a transmitter of thecommunications system to operate cumulatively in a filtering mode; andfiltering a signal using said one or more filters.
 7. The method ofclaim 6, further comprising: determining whether the frequency offset isgreater than a first threshold; wherein the controlling of said one ormore filters is performed upon the determination that the frequencyoffset is not greater than the first threshold.
 8. The method of claim7, wherein the highest frequency in the pass band in said filtering modeis equal to the highest frequency in said allocated band.
 9. The methodof claim 7, wherein said one or more filters includes a first filter anda second filter, the first filter configured to filter an output of thesecond filter either directly or after intermediate amplification, themethod further comprising disabling the second filter responsive to adetermination that an attenuation provided by the first filter at afirst frequency is greater than a first threshold.
 10. The method ofclaim 7, wherein the first and second inputs are received from a basestation.
 11. The method of claim 7, wherein the highest frequency in apass band in said filtering mode is less than the highest frequency ofthe total band available for transmission.
 12. A system comprising: alocal oscillator configured to synthesize a waveform at a variablefrequency; one or more variable bandwidth filters arranged along aserial processing pathway, wherein each filter is capable of beingenabled or disabled; a mixer configured to mix an output of one of theone or more filters based on the waveform synthesized by the localoscillator; and a control module comprising a non-transitory computerreadable storage medium including computer-executable instructionsstored tangibly thereon, said instructions when executed causing aprocessor of the system to perform the operations of: receiving a firstinput and a second input, wherein the first input indicates a frequencyoffset of a frequency band allocated for signal transmission, saidallocated band is a subband of a total band available for transmission,and the second input indicates a bandwidth of said allocated band; andcontrolling the one or more filters of a transmitter of thecommunications system to operate cumulatively in filtering mode.
 13. Thesystem of claim 12, wherein the highest frequency in a pass band in saidfiltering mode is less than the highest frequency of the total bandavailable for transmission.
 14. The system of claim 12, wherein saidinstructions when executed further cause the processor to: determinewhether the frequency offset is greater than a first threshold; whereinsaid controlling the one or more filters is performed upon thedetermination that the frequency offset is not greater than the firstthreshold.
 15. The system of claim 12, wherein each filter is onlyoperable in filtering mode.
 16. The system of claim 12, wherein eachfilter is variably configurable to operate in either bandpass filteringmode or lowpass filtering mode.